Support and decoupling structure for an acoustic resonator, acoustic resonator and corresponding integrated circuit

ABSTRACT

An acoustic resonator assembly includes a layer of high-acoustic-impedance material and a layer of low-acoustic-impedance material made of a low-electrical-permittivity material. This assembly may support the resonator over an interconnect layer or act as a decoupling assembly between two active elements of the resonator. The assembly may alternatively include three low-acoustic impedance layers. Alternatively, the assembly may include three acoustic impedance layers wherein two of the layers are low acoustic impedance layers and the third layer has a higher acoustic impedance than the first two or alternatively is a high-acoustic impedance layer.

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM

The present application is a continuation-in-part (CIP) of PCTApplication No. PCT/FRO3/03500, filed Nov. 27, 2003, which claimspriority from French Application for Patent No. 02 14967 filed Nov. 28,2002, the disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to the field of integrated circuits, andmore particularly to integrated circuits comprising one or more acousticor piezoelectric resonators. Such circuits can be used in signalprocessing applications, for example, for performing a filteringfunction.

2. Description of Related Art

Acoustic resonators are integral with the integrated circuit, whilehaving to be acoustically or mechanically isolated therefrom. For thispurpose, a support capable of producing such isolation may be provided.The support may comprise an alternation of a layer having a highacoustic impedance and a layer having a low acoustic impedance (see,U.S. Pat. No. 6,081,171, the disclosure of which is hereby incorporatedby reference).

The term “acoustic impedance” is understood to mean the quantity Z givenby the density ρ of the material multiplied by the acoustic velocity v,i.e., Z=ρν. The acoustic velocity ν may be taken as being defined by:ν=(ρC₃₃) ₁₂where C₃₃ is one of the coefficients of the elastic compliance matrix.

For high acoustic isolation performance, it is desirable for thedifference in acoustic impedance between the materials to be as high aspossible.

A need exists in the art to meet this requirement.

SUMMARY OF THE INVENTION

The invention proposes a support/decoupling structure for an acousticresonator, providing a high level of acoustic isolation.

The support/decoupling structure for an acoustic resonator, according toone aspect of the invention, comprises at least one assembly comprisinga layer of high-acoustic-impedance material and a layer oflow-acoustic-impedance material made of a low-electrical-permittivitymaterial. It appears in fact that low electrical permittivity goes handin hand with low acoustic impedance. In such a material, an acousticwave propagates slowly.

In one embodiment for decoupling structure between to active elements,the layer of low-acoustic-impedance material is placed between oneactive element and the layer of high-acoustic-impedance material.

In one embodiment, the layer of high-acoustic-impedance material is anupper electrode of said another resonator.

Advantageously, the relative electrical permittivity of thelow-acoustic-impedance material is less than 4, preferably less than 3and better still less than 2.5.

Advantageously, the layer of low-acoustic-impedance material is producedfrom one of the materials used for fabricating the rest of the circuitof which it forms a part, for example, for fabricating the interconnectlevels.

In one embodiment, the low-acoustic-impedance material comprises SiOC.SiOC is a material used sometimes for producing dielectric layers havinga very low permittivity on a substrate or in the interconnects.Preferably, porous SiOC may be used, the acoustic impedance of which iseven lower. The pores of such a material are generally filled with agas, such as argon.

In one embodiment of the invention, the high-acoustic-impedance materialcomprises at least one of the following species: aluminum, copper,nickel, gold, platinum, molybdenum. Copper has an acoustic impedancelower than that of tungsten, but is beneficial as it is often used inthe interconnects of the circuit. A copper layer on the support may thusbe produced during a common step for fabricating interconnects.

In one embodiment of the invention, the layer of low-acoustic-impedancematerial has a thickness of between 0.3 and 0.7 μm.

The invention also proposes an acoustic resonator comprising at leasttwo active elements and a decoupling assembly. The decoupling assemblycomprises at least a layer of low-acoustic-impedance material made of alow-electrical-permittivity material placed between the active elements.

In one embodiment of the invention, an active element comprises at leastone piezoelectric layer placed between two electrodes. A lower electrodeof an upper active element may rest on the decoupling assembly. Thedecoupling assembly may rest on an upper electrode of a lower activeelement. The piezoelectric layer may be made of crystalline aluminumnitride. The decoupling assembly acts as interface between two activeelements.

Advantageously, the decoupling assembly comprises no more than one layerof low-acoustic-impedance material.

Advantageously, the decoupling assembly comprises two layers oflow-acoustic-impedance material.

Advantageously, the decoupling assembly comprises three layers oflow-acoustic-impedance material.

The invention also proposes an integrated circuit comprising asubstrate, a set of interconnects and an acoustic resonator that isprovided with at least two active elements and with a decouplingassembly. The support of the acoustic resonator may include at least onebilayer assembly comprising a layer of high-acoustic-impedance materialand a layer of low-acoustic-impedance material made of alow-electrical-permittivity material.

In one embodiment of the invention, the acoustic resonator is placed onthe set of interconnects, for example being supported by an upperdielectric layer of the set of interconnects.

In another embodiment of the invention, the acoustic resonator is placednear the set of interconnects, the upper electrode of the upper activeelement of the acoustic resonator possibly being flush with the uppersurface of the set of interconnects.

Advantageously, at least one material is common between the support andthe substrate or the set of interconnects. Copper may serve both as thelayer of high-acoustic-impedance material of the support and for themetallization lines of the set of interconnects. Preferably, a commonfabrication step will be provided both for the said layer ofhigh-acoustic-impedance material of the support and the metallizationlevels of the set of interconnects.

A layer of low-acoustic-impedance material may be placed at the samelevel as an interconnect layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 is a schematic view of an integrated circuit according to a firstembodiment of the invention;

FIG. 2 is a schematic view of an integrated circuit according to asecond embodiment of the invention;

FIG. 3 is a schematic view of an acoustic resonator according to oneaspect of the invention;

FIG. 4 is a schematic view of an acoustic resonator according to oneaspect of the invention;

FIG. 5 is a schematic view of an acoustic resonator according to oneaspect of the invention; and

FIG. 6 is a schematic view of an acoustic resonator according to oneaspect of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

As may be seen in FIG. 1, an integrated circuit 1 comprises a substrate2, in which active zones (not shown) are generally formed, and a set ofinterconnects 3 placed above the substrate 2 and in contact with itsupper surface, provided with at least one metallization level allowinginterconnects to be made between the elements of the substrate.

The integrated circuit 1 is completed by a mechanical acoustic resonator4 placed above the set of interconnects 3 in contact with its uppersurface 3 a. The mechanical acoustic resonator 4 supported by the set ofinterconnects 3 will also be provided with electrical connections (notshown).

In the embodiment illustrated in FIG. 2, the acoustic resonator 4 isplaced within the set of interconnects 3 and is flush with its uppersurface 3 a. This construction makes the integrated circuit 1 morecompact. A lower portion of the acoustic resonator 4 may be embodied inthe set of interconnects 3, while an upper portion will be left free soas to be able to vibrate, being separated from the rest of the set ofinterconnects 3 via a groove 5. The groove 5 ensures that the componentis isolated in the lateral directions, that is to say it allows thelayers to vibrate without direct interference with the substrate. Thethickness of the groove 5 may be small, for example less than 1 μm.

The structure of the acoustic resonator 4 will be described in greaterdetail with reference to FIG. 3.

The acoustic resonator 4 comprises an active element 6, a support 7 thatrests on the upper surface 3 a of the set of interconnects 3 andsupports the active elements 6, a decoupling layer 13 that rests on theupper surface of the active element 6 and a supplementary active element14 that rests on the upper surface of the decoupling layer 13.

The active element 6 comprises three main layers in the form of a lowerelectrode 8, a piezoelectric layer 9 and an upper electrode 10. Thesupplementary active element 14 comprises three main layers in the formof a lower electrode 15, a piezoelectric layer 16 and an upper electrode17. The electrodes 8, 10, 15 and 17 are electrically connected (notshown) to conductors provided in the set of interconnects 3. Theelectrodes 8, 10, 15 and 17 are made of conducting material, for examplealuminum, copper, platinum, molybdenum, nickel, titanium, niobium,silver, gold, tantalum, lanthanum, etc. The piezoelectric layers 9 and16 placed between the electrodes 8 and 10, 15 and 17 may be made, forexample, of crystalline aluminum nitride, zinc oxide, zinc sulfide, aceramic of the LiTaO, PbTiO, PbZrTi, KNbO₃ type, or else a ceramiccontaining lanthanum, etc.

The piezoelectric layers 9 and 16 may have a thickness of a few μm, forexample, 2.4 μm. The electrodes 8, 10, 15 and 17 may have a thicknesssubstantially smaller than the piezoelectric layers 9 and 16, forexample, 0.1 μm.

The decoupling layer 13 consists of a single layer made oflow-acoustic-impedance material. In one embodiment of the invention, thematerial of low-acoustic-impedance has a low electric impedance, forexample a material containing SiOC. SiOC or porous SiOC may be used.

The support 7 comprises a high-acoustic-impedance layer 11 resting onthe upper surface 3 a of the set of interconnects 3 and alow-acoustic-impedance layer 12 that supports the lower electrode 8 ofthe active element 6.

The high-acoustic-impedance layer 11 may be made of a dense material,such as amorphous aluminum nitride, copper, nickel, tungsten, gold ormolybdenum. Alloys or superpositions of sublayers of these species maybe envisaged. Tungsten offers an extremely high acoustic impedance andmay be obtained so as to avoid the residual fabrication constraints,especially in a xenon environment, for example, by a xenon plasma.Copper offers less favorable acoustic impedance characteristics thantungsten, but has the advantage of often being used in the sets ofinterconnects for forming the conducting lines. Its use in thehigh-acoustic-impedance layer 11 may allow the said layer 11 to beproduced by the same fabrication step as that for the conducting line ofthe set of interconnects, which is particularly economic.

The low-acoustic-impedance layer 12 is made of a material having a lowelectrical permittivity, because of the correspondence between lowelectrical permittivity and low acoustic impedance. The permittivity ofthe material of the layer 12 is less than 4. However, it will bepreferred to use a material having a permittivity of less than 3, forexample, a dielectric having a permittivity of around 2.9, often used asdielectric layer in the active zones of the substrate or in the set ofinterconnects 3. Here again, the same fabrication step may be used toform the layer 12 and a dielectric layer of the set of interconnects 3.For example, SiOC or SiOC-based material may be used. It is even moreadvantageous to make the layer 2 from a material having an ultralowpermittivity of less than 2.5, for example around 2.0. For this purpose,the layer 12 may be made of porous SiOC or may be based on such amaterial. The pores of the porous SiOC may contain argon.

It will be understood that it is particularly advantageous from aneconomic standpoint to produce the support 7 from chemical species usedfor the fabrication of the set of interconnects. It is then possible toprofit from the fabrication steps for the said set of interconnects inorder to produce the support 7. This therefore avoids additional stepsand a longer fabrication process.

Since the low-acoustic-impedance material of the layer 12 offers a verylarge acoustic impedance difference relative to that of the layer 11,the acoustic and/or mechanical isolation provided by the layer 7 betweenthe active element 6 and the rest of the integrated circuit is improved.As a result, it is possible to reduce the number of pairs of layers 11and 12 of the layer 7 for the same isolation characteristics. Thus, anapplication conventionally requiring three or four pairs of layers maybe produced with only one or two pairs of layers 11 and 12, hence makingthe acoustic resonator more compact and reducing the costs. FIG. 3 showsa support 7 with one pair of layers 11 and 12. However, it is possibleto provide a support 7 with two superposed pairs of layers 11 and 12, oreven three or more pairs of layers 11 and 12, which then gives acousticisolation characteristics of very high level.

It should be noted that a reflector may comprise an odd number of layersif a first layer of low acoustic impedance is placed under one or morebilayers.

The thickness of the low-acoustic-impedance layer 12 depends on theresonant frequency of the active element 6 and could advantageously bearound one quarter of the wavelength. The layer 12 may have a thicknessof the order of a few tenths of a micron, preferably less than 0.7 μm,for example from 0.2 μm to 0.7 μm. The thickness of thehigh-acoustic-impedance layer 11 may be the order of a few tenths of amicron, for example 0.3 μm to 3.2 μm.

The low-acoustic-impedance material of the decoupling layer 13 offers avery large acoustic impedance difference relative to that of theelectrodes 10 and 15 comprising a high-acoustic-impedance material. Agood acoustic isolation between the active elements 6 and 14 isobtained.

The invention therefore offers a support for an acoustic resonatorhaving a very high acoustic impedance of between 30×10⁻⁶ and 130×10⁻⁶kg/m².s. It is thus possible to benefit from an acoustic resonator, andfrom an integrated circuit, that is more compact and more economicbecause of the reduction in the number of layers.

In the embodiment illustrated in FIG. 4, the decoupling layer 13 isreplaced by a decoupling assembly 18 placed between the upper electrode10 of the lower active element 6 and the lower electrode 15 of thesupplementary active element 14. The decoupling assembly 18 comprisestwo layers 19 and 20 of low-acoustic-impedance material. The acousticimpedance of the layers 19 and 20 are different so as to obtain animproved decoupling effect. The low-acoustic-impedance layers 19 and 20may both comprise SiOC material. For example, the low-acoustic-impedancelayer 19 consists of porous SiOC comprising argon and thelow-acoustic-impedance layer 20 comprises non-porous SiOC or anon-porous SiOC-based material having an acoustic impedance higher thanthe acoustic impedance of the layer 19. The decoupling assembly 18provides an improved acoustic decoupling between the active element 6and the supplementary active element 14.

In the embodiment of FIG. 5, the decoupling assembly 18 comprises asupplementary upper layer 21 placed between the layer 20 and the lowerelectrode 15 of the supplementary active element 14. At least one of thelayers 19, 20 and 21 has an acoustic impedance different from theacoustic impedance of the two other layers. For example, the layers 19and 21 are made of ultralow acoustic impedance material. It isadvantageous to make the layers 19 and 21 of porous SiOC or of porousSiOC-based material. The layer 20 has an acoustic impedance greater thanthe acoustic impedance of the layers 19 and 21. For example, the layer20 comprises non-porous SiOC or non-porous SiOC-based material.

In another example, the layer 20 is made of high acoustic impedancematerial, such as the material used for an electrode of an activeelement or the high-acoustic-impedance material of the layer 11 of thesupport 7. The layers 19 and 21 may be made of the same ultralowacoustic impedance material or may be made one of an ultralow acousticimpedance material and the other of a low-acoustic impedance material.

The invention therefore offers a stack of resonators acousticallyseparated by decoupling layers or assemblies. Consequently, the acousticresonators are particularly compact and economic.

In the embodiment of FIG. 6, the acoustic resonator comprises threeactive elements 6, 14 and 22, a support 7 and two decoupling assemblies18 and 21. The active element 6 rests on the support 7. The decouplingassembly 18 is placed between the active elements 6 and 14. Thedecoupling assembly 21 is placed between the active elements 14 and 22.The decoupling assembly 21 may be similar to the decoupling assembly 18.The active element 22 may be similar to the active elements 6 and 14. Anacoustic resonator comprising three active elements is compact whilehaving high decoupling properties between the active elements.

Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

1. A decoupling structure, comprising: at least one assembly positionedbetween first and second acoustic resonators, comprising: a layer ofhigh-acoustic-impedance material; and a layer of low-acoustic-impedancematerial made of a low-electrical-permittivity material.
 2. Thestructure according to claim 1, wherein the layer oflow-acoustic-impedance material is placed between one of the twoacoustic resonators and said layer of high-acoustic-impedance material.3. The structure according to claim 1, wherein the layer ofhigh-acoustic-impedance material is made of a same material as anelectrode of one of the acoustic resonators.
 4. The structure accordingto claim 1, wherein the low-acoustic-impedance material comprises SiOC.5. The structure according to claim 1, wherein thelow-acoustic-impedance material comprises at least 90% SiOC in weight.6. The structure according to claim 1, wherein thelow-acoustic-impedance material comprises porous SiOC.
 7. The structureaccording to claim 1, wherein the layer of low-acoustic-impedancematerial comprises a plurality of sublayers comprising SiOC.
 8. Thestructure according to claim 1, wherein the high-acoustic-impedancematerial comprises at least one material selected in the groupconsisting of: aluminum, copper, nickel, gold, platinum, molybdenum. 9.An acoustic resonator, comprising: at least two active elements; and adecoupling assembly placed between pairs of active elements comprisingat least a layer of low-acoustic-impedance material made of alow-electrical-permittivity material.
 10. The resonator according toclaim 9, wherein an active element comprises at least one piezoelectriclayer placed between electrodes.
 11. The resonator according to claim 9,wherein the decoupling assembly is placed between an upper electrode ofa first active element and a lower electrode of a second active element.12. The resonator according to claim 9, wherein thelow-acoustic-impedance material comprises a SiOC material.
 13. Theresonator according to claim 9, wherein an electrode comprises at leastone material elected in the group consisting of: aluminum, copper,nickel, gold, platinum, molybdenum.
 14. The resonator according to claim9, wherein the decoupling assembly comprises no more than one layer oflow-acoustic-impedance material.
 15. The resonator according to claim 9,wherein the decoupling assembly comprises two layers oflow-acoustic-impedance material.
 16. The resonator according to claim 9,wherein the decoupling assembly comprises three layers oflow-acoustic-impedance material.
 17. An integrated circuit, comprising:a substrate; a set of interconnects; and an acoustic resonator that isprovided with at least two active elements and with a decoupling layerpositioned between each pair of active elements.
 18. The circuitaccording to claim 17, wherein the acoustic resonator is placed on theset of interconnects.
 19. The circuit according to claim 17, wherein theacoustic resonator is placed near the set of interconnects.
 20. Thecircuit according to claim 17, wherein a first acoustic resonatorcomprises a support on an interconnect layer, the support including atleast one bilayer assembly comprising a layer of high-acoustic-impedancematerial and a layer of low-acoustic-impedance material made of alow-electrical-permittivity material.
 21. The circuit according to claim17, wherein the decoupling layer comprises at least a layer oflow-acoustic-impedance material.
 22. The circuit according to claim 17,wherein the low-acoustic-impedance material comprises a SiOC material.23. The circuit according to claim 20, wherein thehigh-acoustic-impedance material comprises at least one materialselected in the group consisting of: aluminum nitride, copper, nickel,tungsten, gold, platinum, molybdenum.
 24. A decoupling structure,comprising: at least one assembly positioned between a first and secondacoustic resonators, comprising: a first layer of low-acoustic-impedancematerial having a first acoustic impedance; and a second layer oflow-acoustic-impedance material having a second acoustic impedance. 25.The structure according to claim 24, wherein the first layer oflow-acoustic-impedance material comprises a layer of SiOC.
 26. Thestructure according to claim 25, wherein the first layer oflow-acoustic-impedance material comprises a layer at least 90% SiOC inweight.
 27. The structure according to claim 25, wherein the secondlayer of low-acoustic-impedance material comprises a layer of porousSiOC.
 28. A decoupling structure, comprising: at least one assemblypositioned between a first and second acoustic resonators, comprising: afirst and second layers of low-acoustic-impedance material having afirst acoustic impedance; a third layer of low-acoustic-impedancematerial having a second acoustic impedance.
 29. The structure accordingto claim 28, wherein at least one of the layers oflow-acoustic-impedance material comprises a layer of SiOC.
 30. Thestructure according to claim 29, wherein the at least one layer oflow-acoustic-impedance material comprises a layer at least 90% SiOC inweight.
 31. The structure according to claim 29, wherein at least oneother layer of low-acoustic-impedance material comprises a layer ofporous SiOC.
 32. The structure according to claim 28, wherein the thirdlayer is a middle layer between the first and second layers.
 33. Thestructure according to claim 32, wherein the second acoustic impedanceis higher than the first acoustic impedance.
 34. A decoupling structure,comprising: at least one assembly positioned between a first and secondacoustic resonators, comprising: a first layer of low-acoustic-impedancematerial having a first acoustic impedance; a second layer oflow-acoustic-impedance material having a second acoustic impedance; alayer of high-acoustic-impedance material.
 35. The structure accordingto claim 34, wherein at least one of the first and second layers oflow-acoustic-impedance material comprises a layer of SiOC.
 36. Thestructure according to claim 35, wherein the at least one layer oflow-acoustic-impedance material comprises a layer at least 90% SiOC inweight.
 37. The structure according to claim 35, wherein the other ofthe first and second layers of low-acoustic-impedance material comprisesa layer of porous SiOC.
 38. The structure according to claim 34, whereinthe third layer is a middle layer between the first and second layers.39. The structure according to claim 34, wherein the second acousticimpedance is higher than the first acoustic impedance.
 40. The structureaccording to claim 34, wherein the first and second acoustic impedancesare the same.
 41. An acoustic resonator, comprising: a first, second andthird active elements; a support positioned under the first activeelement comprising a first high acoustic impedance layer and a first lowacoustic impedance layer; a first decoupling assembly positioned betweenthe first and second active elements comprising plural layers ofacoustic-impedance material; and a second decoupling assembly positionedbetween the second and third active elements comprising plural layers ofacoustic-impedance material.
 42. The resonator of claim 41 wherein atleast one of the first and second decoupling assemblies comprises: afirst layer of low-acoustic-impedance material having a first acousticimpedance; and a second layer of low-acoustic-impedance material havinga second acoustic impedance.
 43. The resonator of claim 41 wherein atleast one of the first and second decoupling assemblies comprises: afirst layer of low-acoustic-impedance material having a first acousticimpedance; a second layer of low-acoustic-impedance material having asecond acoustic impedance; and a layer of high-acoustic-impedancematerial.